Kites for Yachts.

About once a month on average I'm contacted by someone who has the idea of using kites in place of, or as well as, conventional sails for larger (that is, offshore capable) yachts.
Attaching kites to conventional yachts, mono or multihull may be part of the development path but will not be the eventual solution. Sooner rather than later, boats will be purpose designed for kite power.

Background.

I believe utterly that kite sailing will eventually become practicable and common but there are considerable technical challenges to overcome before this can happen.
The history of kitesailing goes back 1000's of years (Hawaiians were using kites to pull canoes between Islands long before European contact) and the first known treatise on upwind sailing using controllable kites was written in the 1820's by George Pocock, the father of kite traction. Pocock used four line framed kites and is known to have taken a group of friends and family kite sailing on the Bristol Channel.
S.F. Cody crossed the English Channel under kitepower in 1902 and Ian Day set a class sailing speed record with a Flexifoil stack powered Tornado in 1978.
A late-comer to the sport, starting in 1987, I've now put in much more than 1000 hours on the water sailing boats under kite power, have developed and tested numberless kites and well over 100 purpose built boats in various configurations (but nothing longer than 8m). Many others have put in the same or more effort, but kite sailing has not yet become an established activity.

Why use kites rather than sails?

The biggest advantage of kite power is that pull can be applied to the craft so as to minimise heeling. Theoretically this eliminates the need for a keel or multi-hull type form stability. In practice however, waves can be as big a threat to stability as wind loads on sails, so stabilisation of some sort (keel or multiple hulls) is still required on dedicated kitesailing craft in open waters except maybe for very large craft (say bigger than 20metres).

Water Launching.

Most newcomers to the field of waterborne kite traction focus on launch and water relaunch ability as a key problem, but there are now many different practical relaunch systems: Self relaunching kites of the arch style (LEI and ram air), mast assisted launching (used with Outleaders), pilot kite assisted launching, and motor assisted launching (a reliable option for any boat that has auxiliary power). Another possibility is kites that launch and fly de-powered on a single line - and when up and settled, can convert to multi-line form and power up.

Lighter than air kites

For launching and to completely solve the problem of how to keep a kite up when there's no wind, helium inflation is often suggested. Practical lighter than air kites may be developed eventually but there are many problems with this approach:
Helium is very expensive, and even with the special (and heavy) fabrics that are used for gas envelopes, replenishment can still be required depressingly often.
Because of the requirement to use relatively thick fabric, lighter than air structures have to be quite large before they contain sufficient volume of helium to offset their weight. The optimum shape is a sphere, (least surface area ,and hence fabric weight, for the volume), but, to fly efficiently, kites need to be wide thin and flat. Therefore lighter than air kites have to be quite large to work at all (minimum 50sq.m's?), and even larger still if they are to have useful upwind capability. This suggests that the lighter than air is approach is only applicable for larger yachts and commercial shipping. These are major niches though.
Perhaps their greatest difficulty is what to do with any lighter than air traction kite when a serious squall hits- seems like the only option would be to dump the helium to atmosphere.

Efficiency.

A serious impediment to kitesailing is that the best traction kites available now attain an L/D (Lift to Drag ratio, the measure of upwind performance) of around eight,- but kites with this peak of performance currently tend also to be too twitchy and unstable for most conditions. Kites that are adequately reliable for general use currently have L/D's of 5 to 6.
Conventional yacht sails can have L/D's in excess of 10.
The effect of this is that kite powered sail boats are not very competitive upwind - sometimes taking more than twice as long as equivalent conventionally rigged craft on a given upwind course. For very high performance craft that are 'on the wind' even downwind, this deficit applies on all courses to some extent- though by using long lines and 'figure eighting' the kite while steering the boat straight downwind, it is usually possible for kite powered craft to prevail against everything else on reaches and runs- and often by more than a 2/1 margin on time.
In the last decade or so, kite performance has been improving much more rapidly than sail performance has though, so there might not always be such an L/D deficit. Since 1980, useable traction kite L/D has improved by a lot, and there is a real possibility of dramatic improvement-hopefully to something approaching parity with conventional sails- within the next few years. Most promising hope are the aeroplane form kites currently being developed for kite energy projects, Rigid carbon fibre structures, they have huge depower, excellent control (by servo controlled surfaces) and should be able to achieve L/D's above 15. Notwithstanding their high weight/area ratio, because of their high L/D, they can fly in very light winds provided they are constantly figure-eighted.

First Law of Kitesailing.

Which leads to a further problem for kitesailing, which is that; the kite has to stay flying. Masts hold sails up even when there is no wind, but kites fall into the water. Apart from the already mentioned, (and to date impractical), lighter than air kites, at less than 8km/hr wind speed, kites that are generally useful in traction applications will not stay up reliably. Apart from when the wind is just not strong enough, there are various other occasions when such light wind is encountered:
One is when the kite is overflown and drifts back, stalled and windless until it is can re-engage. This may happen even when the true wind speed is quite a bit stronger than the 8km/hr minimum, but can be controlled by user friendly kite design and skillful flying.
Another is downwind sailing when the boat over-runs the kite. Never totally avoidable, careful coordinated control of boat and kite does help a lot though- and it's mainly a problem in light winds that are 'technical' rather than fun to sail in anyway. Downwind overflying is also more of a problem at the performance end of kitesailing; heavy slow boats using small kites don't have this problem.
The most annoying is when the wind is gusty (which is almost always for inshore locations) with periods when the actual wind speed is less than the required minimal. Up to 30 seconds or so of lull can usually be bridged, especially if the boat has good momentum and can be turned upwind a bit to keep the kite engaged with some apparent wind until the lull passes. Fortunately also, the highly developed kites used now for small boat kitesailing can generally be relaunched in the next puff even after they've fallen onto the water because of a lull- but on average, kitesailing is not currently possible unless the average wind is more than 8km/hr, and remains frustratingly difficult unless the wind is consistently more than 10km/hr.
A distressing aspect of light wind kitesailing therefore is that conventional sailboats can drift along still making ground long after those sailing under kitepower are retrieving their dripping kite and lines from the water-or from the underwater appendages of some other boat.

Making Bigger Kites.

As kites are made larger, for structural reasons, they must necessarily become heavier in proportion to their size-which causes large kites to perform worse in light winds than smaller kites. A "magic" number is 200gms/sq.m of projected area. Kites that weigh much more than this generally handle poorly- and not just in light winds either. Aerodynamically literate non-kite-fliers find this almost impossible to believe- they 'do the numbers' and determine that kite weight is such an insignificant proportion of kite pull in even marginal winds that it can't be a factor. It is though.
Framed kites (of the delta style for example) are severely limited by this scaling effect (with current materials)- to about 15sq.m.
Hybrid style traction kites, like C Quad's for example, are less limited but would certainly become too unwieldy over about 30 sq.m.
LEI's (inflated leading edge single skin kiteboarding style kites) are less limited and can certainly be taken to 100sq.m, probably 300sq.m, not least because their performance appears to be less effected by weight than other styles (because their substantial camber allows them to fly at a higher angle of attack then other styles?).
Ram air inflated kites of the bridled or Arc type are better still and can probably scale to 500sq.m or more before weight/area becomes restricting, perhaps quite a bit more.
NASA style frameless single skin multi-bridle kites could be built to at least 250sq.m and probably to 1000sq.m or more but their L/D (+/-3) limit's their usefulness to reaching and downwind courses when sailing against conventionally rigged yachts (although because they fly so well in very light winds, they can be competitive upwind against high performance kites in very light winds).
Outleader style single skin frameless 4 bridle kites of 450sq.m are completely successful. 1000sq.m seems possible without their becoming either too fragile or unmanageable, maybe even more. Unfortunately their L/D is lower even than for the NASA's, making them incapable of useful upwind sailing.

The biggest challenge.

The biggest challenge in the development of kitesailing is the, variously called, max./min. problem.
This problem is caused by the maximum kite pull for any given wind being 5 to 25 times that of a similar sized conventional sail in the same conditions, while it's average pull will be about the same.
20 sq.m kites are generally flown on lines of 30m or so and larger kites require even longer lines- whatever the minimum length is to get the kite clear of turbulence, to give it room for manoeuvre, and sufficiently above the waves to reduce the likelihood of accidental immersion. At these line lengths, kites will at times accelerate to a maximum apparent wind speed that is almost totally independent of boat speed. In fact, their maximum speed through the air in any given circumstances will be near enough to the true wind speed multiplied by the kite's L/D ratio.
For example, in a true wind of 20km/hr and for a kite of L/D 5, max. kite speed will be 100km/hr- at which speed it will theoretically develop 25 times as much pull (it's a square relationship) as it will at steady state in 20km/hr, or 11 times the pull at 30km/hr, the likely apparent wind speed for an upwind course on an average sailboat in 20km/hr true. Actually in practice it's not quite this vicious- but is still quite bad enough to make kitesailing difficult and potentially dangerous. It is almost impossible to design a kite, rigging, attachment system and even to build the boat itself strongly enough to take this level of overpull without breaking.
One answer is to just undersize the kite by a large factor so that it's max. pull will be within line and boat limits- but then the average performance will be dismal. A principle of competitive kitesailing is that the kite pull in steady state on-the-wind conditions (that is, say, 30km/hr apparent in a 20km/hr true wind) must be approximately equal to the 'lift' generated by a correctly chosen suit of sails in similar conditions. If the kite has less pull in this comparison, upwind performance will be even slower than the poorer L/D performance of kites will already have caused.
However, by very careful and skilled kite flying it is usually possible to prevent the kite 'getting away' to its max. apparent wind speed. The operative word here, unfortunately, is 'usually'. In turbulent conditions, through momentary inattention or in even minor emergencies it is inevitable that the kite will occasionally accelerate to its max. speed/max. pull. In anything except zephyr winds, the result of this will be structural failure of the kite, line, rigging or boat, with considerable risks to crew- or the boat itself will be catapulted into the air, with no certainty of coming down in one piece or necessarily the right way up. Even on an 8m kiteboat I have done leaps of 10metres or more and have heard of 50m. Because of the requirement, that to be competitive, larger boats must have average kite pull in proportion to their size, they are not immune to this effect.
There are some ways to mitigate this max/min problem other than by perfect kite control though:

Gust Response.

One is to build automatic de-powering systems into the kite. Automatic de-powering is generally called gust response and the framed delta style traction kites we used for kitesailing in the late 1980's were intrinsically very good in this respect; their pull increased at only about half the rate of the underlying theoretical square relationship between wind speed and pull.
Current Arc style soft kites have good gust response.
Bridled 'foils and NASA style single skin frameless kites have almost no gust response.
LEI (leading edge inflatable) kiteboarding kites have some gust response.
Aeroplane style rigid kites (including 'Box' kites) can have excellent gust response if the angle between their front and rear surfaces is made to be proportional to line pull.
Very many automatic pull mitigation systems have been tried in the more than 175 years during which kites have been developed for sailing applications, but except for suitably rigged aeroplane style kites are inherently better than framed delta style kites in this respect have yet proven to be satisfactory in practice. A problem with many of the systems that have been tried is surging or pumping; Harmonic interaction between aerodynamic forces and flexible or responsive elements. This, at least, costs performance and is unsettling for the sailor(s)- but can be destructive and even dangerous if a runaway increase in the amplitude of surging occurs.

An advantage for lower performance kites.

A limited answer to overpull is to use kites with lower efficiency- because their maximum pull will be only 5 to 10 times their average rather than the 25 times or more (mitigated by state of the art pull control) of high performance kites. Examples are NASA wing style soft kites and the Outleader style developed by Dave Culp/Dean Jordan, for the 2003 America's Cup contest These kites are only useful when sailing downwind and on broad reaches. NASA and Outleader style kites are however generally useable even on boats that have not been specifically designed for kitesailing- and the Outleader has developed some market as a spinnaker replacement.

In very light winds, the NASA type has useful upwind performance when compared to other styles of kites that are used for kitesailing. The reason for this is that, because of their highly cambered form, NASA's develop strong pull even while hanging at their maximum angle to the edge even in very light wind- whereas higher performance kites require to be 'figure eighted' in these conditions, thereby losing much of their effective upwind angle.

Safety Release Systems.

A total but sometimes inconvenient answer to the overpull problem is to build in an automatic (and/or manual) release system that either releases the kite completely (not so safe for anyone downwind though and requiring later recovery in any case) or half releases the kite so that it will collapse on to the water while still attached to the boat. Release systems work provided they don't have to be used too often- the idea is not to spend half or more of your total sailing time doing retrievals and relaunching- but are a useful and necessary fall back when used in conjunction with other overpull mitigation systems.

Flier Control of Pull.

The other major way to mitigate the max/min problem is volitional pull control, usually called de-power.
By this, the kite's flier has some control over how much pull the kite will have at any instant. It is generally accomplished by changing relative line lengths- conventionally by reducing the tension on rear lines- but can be by other means. For single line traction kites (such as the Arc used by Delft University in their kite energy program) it is by servo control, for aeroplane style kites, de-power is effected by changing the angle between the main wing and tailplane.
For kite boarding kites, power control is often called 'sheeting'- a term borrowed from sailing.
Until about 2005, the maximum de-power generally available was around 30%- that is, when a kite is sheeted to max. power, the flier could choose to reduce it's pull to 70% or a little more. Each of the major styles of traction kite has been vying for the lead in this prized characteristic, but LEI's are currently the clear leader with their 'Bow' or 'supported leading edge' style and 5th line systems. Bow kites can be de-powered by as much as 75%, albeit with significant loss of control towards this end. 5th line LEI's depower nearly 100%, but are not steerable when flown off their 5th (de-power) line.

Is Blue Water Kite Sailing going to happen soon?

The best things that have happened recently for kite sailing are kite boarding and the large investment currently going into the development of kite energy systems. Spin offs from kiteboarding have already advanced kite sailing a lot, and kite energy research will surely overflow also.
The work on autonomous flying systems- kite auto pilots- could have some applications (though I remain skeptical about practicality in anything except smooth mid range and stronger winds, which have been rare to non-existent in my kite sailing experience to date).
What will definitely advance the cause of kite sailing is the ongoing development of new and existing kite styles, and their control and winch systems, that kite energy projects have been sponsoring.
The single line servo controlled bridled 'foil developments by Skysail in Hamburg is also exciting. It seems to me that their systems have immediate application to recreational sailing, while not having much if any chance of unsubsidised viability in their chosen target of commercial shipping.
The pressure of all this development will sooner or later bring about a break through.
Probably, all the elements we need already exist, it will just take someone to put them together in the right combination and convince the world it's worth doing.

Boats for Kitesailing.

My view is that boats should be specifically designed for kitesailing, that adapting conventional sailboats (mono or multihull) is about as satisfactory as converting a power boat for sailing would be. Sure, it can be done but can never be satisfactory, let alone optimal. The fundamental problem with using monohull sailing boats with kite power is that they are primarily designed to resist heeling moment. They have weighted keels-which can be much more than half the boat's weigh, and every aspect of their hull design is optimised towards maximum righting moment with minimum drag while heeled. Kite powered boats don't heel (if their kite attachment geometry is correct) so they don't need keels. Nor do they have any tendency to nose dive if the kite pull is appropriately located, but conventional yacht hull shapes are determined substantially by the need to resist sail forces operating some metres above their decks. Monohull kite boats can be narrower and longer than any sail boat ever built-and they'll be faster because of this. Multihulls (cats and Tri's) resist heeling loads by form stability. There are two performance costs with this. Firstly there is substantial wave interference between their hulls. Secondly, the structure connecting their hulls is a substantial part of their weight (and cost). Sailing Multihulls are also subject to catastrophic pitchpoling, and their hull forms are consequently distorted by the requirement to have maximum flotation forward. This has considerable aerodynamic and hydrodynamic costs that are neither necessary nor useful for a kite powered craft. Kite boats should be long, narrow, and have only one hull. They need not and should not have any unnecessary aero drag. This advantageous form can more than compensate for the current disadvantages of kite power when compared with conventional sails (relatively lower L/D and inability to sail in very light winds). The use of this optimal hull form is made possible by the absence of heeling moment when using kite power. The other inherent advantage of kite power, that kite pull is not limited by the boat's apparent wind speed, but can be five or more times higher, will then assure the kite boat's superiority over conventional sailing craft in competitive events.

Next Steps:

The above is far from a comprehensive or rigorous explanation of kite sailing practicalities but at least it introduces some of the relationships. Designing for kite sailing is rather like trying to design a modern motorcar in one step without having ever seen or driven one- and about as likely to succeed, that is, zero chance. Jumping straight on to a large kitesailing boat is equivalent to taking your first driving lesson solo on the motorway at rush hour. Indirect flying with a high performance kite is very much more difficult than when you have full personal control of the kite.
Thinking long and hard, dreaming and planning are no substitutes for practical experience.
One way to begin is to get a KiteCat and go kite sailing. Time on the water in a purpose designed specialist boat is a fast way to get an understanding of kite sailing- and it's fun to do.
Alternatively, align yourself with some person or group involved with kite sailing development- there are a lot- and get experience this way.
There really is now no technical impediment standing in the way of kite sailing becoming practical and mainstream, so if you really have the pioneer spirit, get into it now- you might be the one credited later with having truly started it all. Someone's going to be!